The Light and Water Stress Tolerance of Two Invasive Legumes: Cytisus scoparius (Scotch broom) and Spartium junceum (Spanish broom)
The ability of Cytisus scoparius L. and Spartium junceum L. to acclimate to different levels of light and water stress was studied to understand ecological constraints on distributions of these plants. A reaction norm experimental design was used to determine growth and physiological responses of each to imposed conditions.
Light treatments were stressful for both species. Increases in shade led to decreases in relative growth rates (RGR); however, light was more of a limiting factor for S. junceum because of the greater decrease in its growth rate at lowest light intensities. As light decreased, stem allocation increased in S. junceum. More differences in allocation to leaves and roots among light treatments were found in C. scoparius. Correspondingly, the greatest changes in photosynthetic characters were found in S. junceum stems and C. scoparius leaves. Differences in physiological traits did not prove to be acclimation to low light levels because quantum yield decreased when light decreased. Neither species exhibited rapid growth rates, normal allocation patterns, or proper adjustments of photosynthetic characters under light conditions below 50% full intensity.
Water treatments did not cause any critical changes in growth or physiology of either species. Neither species' RGR was greatly decreased. Water availability is more of a determining factor of growth for C. scoparius because of its continued, small decline in RGR as watering frequency decreased. Spartium junceum appeared to be better adapted to the imposed water treatments with its smaller number of leaves, leaf surface area, percentage leaf weight, specific leaf area, and leaf area / stem area ratio. Spartium junceum also displayed larger root / shoot ratios in drier conditions. The magnitude of these shifts was typically greater in C. scoparius, but the RGR of this species decreased more than that of S. junceum under drier conditions. More stress was imposed on C. scoparius because its lowest water potential measurements were 40% lower than those of S. junceum and below the water potential value at turgor loss point during midday hours. When water potential decreased, Spartium junceum showed signs of acclimation because stem photosynthesis increased and leaf photosynthesis decreased while C. scoparius did not adjust its rates of photosynthesis. Water treatments did not impose enough stress to cause osmotic adjustments. Performance under these light and water treatments explained the habitat preference of each to areas where these species have become invaders along the Pacific Coast of the Unites States.